2 Wire Temperature Sensor

2-Wire Temperature Sensor Circuit Diagram. The Type LM35 temperature sensor from National Semiconductor is very popular for two reasons: it produces an output voltage that is directly proportional to the measured temperature in degrees Celsius, and it enables temperatures below zero to be measured. A drawback of the device is, however, that in its standard application schema it needs to be connected to the actual measuring schema via a three-wire link. This drawback is neatly negated by the present schema. When the LM35 is connected as shown, a two-wire link for the measurement range of –5 °C to +40 °C becomes possible.

2-Wire Temperature Sensor Circuit Diagram:

  

Two-Wire Temperature Sensor Circuit Diagram

Actually, the schema shown is a temperature-dependent current source, since it uses the variation of the quiescent current with changes in temperature. The values of resistors R3 and R4 are calculated to give an output voltage of 10mV °C–1. Where good accuracy is desirable or necessary, 1% resistors should be used. In this context, note that a loss resistance in the link between sensor and measuring schema may cause a measurement error of about 1 °C for every 5 ohms of resistance. Capacitor C1 eliminates undesired interference and noise signals. At an ambient temperature of 25 °C, the schema draws a current of about 2mA.